Dividends on common stock: (Details) - USD ($) $ / shares in Units, $ in Thousands			6 Months Ended
	Sep. 03, 2021	Aug. 04, 2021	Jun. 30, 2021	Jun. 30, 2020
Dividends on common stock and return of capital program
Dividends paid			$ 73,082	$ 62,295
Subsequent Event
Dividends on common stock and return of capital program
Quarterly dividend payment approved (per share)		$ 0.805
Estimated
Dividends on common stock and return of capital program
Dividends paid	$ 37,200